Acoustic well logging methods and systems have been extensively described in the art. In a typical system an acoustic transmitter is located above a pair of sonic receivers on a well logging tool. As the tool is pulled upwardly, the transmitter is actuated to direct acoustic energy into the borehole. As a result of the transmitter's cylindrical shape, most of its energy is directed normal to its axis and radially outward towards the surrounding earth formation where it subsequently travels both upward and downward along the borehole wall as well as outward in the formation. The receivers, located below the transmitter, utilize only the part of the energy traveling downward along the borehole and through the formation. The energy traveling upward and outward is, consequently, not utilized. This inefficient energy use limits spacing allowable between the transmitter and receivers. Energy received by a receiver located too far from a transmitter would be too low to allow reliable processing of received waves to derive information such as the travel time of a particular wave, or the magnitude of the wave, or such other parameter which leads to an understanding of the earth formation around the borehole as a function of depth.
The magnitude of the waveforms incident upon the receivers is a function of a large number of factors such as attenuation of the waveforms due to the transmitter to receiver spacing, the character of the formation, the dominant frequency spectrum of the acoustic energy, borehole shape and size. Also, formations may be acoustically altered near the borehole such that signals from receivers located at short transmitter-to-receiver spacings do not represent the radially deeper unaltered formation characteristics. To make accurate measurements under such conditions requires longer spacings and stronger signals at these distant receivers. In light of the variable nature of the earth formation, the magnitude of the acoustic energy incident upon the receivers frequently tends to be marginal.
Proposals have been made to improve the magnitude of the acoustic waves traveling downward toward the receivers by controlling the directivity of the energy distribution from the transmitter. One technique, such as illustrated in U.S. Pat. No. 3,136,318 issued to Anderson on June 9, 1964, alters the directivity pattern in a fixed manner. In other techniques the directivity pattern may be mechanically adjusted, as described in U.S. Pat. No. 3,542,150 issued to Youmans; or electrically adjusted such as by the technique described in U.S. Pat. No. 3,496,553 issued to Semmelink. These adjustments are usually made prior to running the borehole tool into the borehole to perform the logging and are consequently based upon estimated logging conditions. The electrically adjustable techniques usually employ a technique known as beam steering, involving a delayed firing of the transmitters so that the overall energy pattern has a peak in a particular direction.
As described in the Semmelink patent, a transmitter is formed of a plurality of separate elemental generators which are closely spaced from each other along the axis of the borehole tool. These generators are electrically connected in series through electrical delay lines. When the transmitter is fired, the uppermost generator is pulsed first with the pulsing of each lower generator delayed relative to the one above it. Delays are such that the pulsing of each element generates an acoustical wave which is in phase with the previously generated waves traveling downward. This phase delayed pulsing generates an overall acoustical pulse whose energy distribution is particularly enhanced in a direction from the first toward the last pulsed element of the transmitter. This directivity changes both with the physical distance between the elements and the internal delay. The physical distance is set on an assembly of the transmitter. The electrical delay could be changed in the field by changing the charging voltage or capacitors used in the electrical circuits.
In U.S. Pat. No. 3,732,945 to Lavigne, a transmitter arrangement is described whereby the directivity of energy can be changed by phase delaying the firing times of a plurality of transmitters. The delay times are fixed under control of a signal set at the surface. By setting the control signal for different delays, specific waves, such as the compressional or shear wave, in the acoustic wave arriving at the sonic receivers, can be emphasized by controlling the directivity of the acoustic energy at the transmitters.
Still another sonic well logging technique using electrically selectable beam steering is described in U.S. Pat. No. 3,614,725 to Moran. As described in this patent, the downward directivity angle of an acoustic transmitter beam may be varied remotely such as from the earth surface by setting a magnetic bias on permeable cores interconnecting separate transmitter elements.
Other well logging techniques using beam steering at a set angle or resettable angle have been described, such as found in the U.S. Pat. No. 3,475,722.
The well logging technique using beam steering as it is described in the above prior art does not provide for an adjustment to frequent variation as may be encountered by a well logging tool when it traverses different formations. In fact, a common practice with well logging techniques having a beam steering capability is to select a fixed beam angle based upon best estimates of the formation characteristics rather than actual measurements and maintaining this fixed angle through a single well logging pass. Though such technique can be a useful application of beam steering, the well logging results may not utilize the optimum potential of beam steering. Further, the relative physical position of the transmitter and receivers in the prior art was not suitable in normal operations to allow proper use of actual measurements to make steering adjustments.